Beta-phase silicon carbide is a material which can withstand high temperatures, has good thermal shock resistance, good abrasion resistance, good corrosion resistance and maintains its strength well at high temperatures. Beta-phase silicon carbide therefore has excellent potential as a base material for the production of pressureless sintered components which are required to withstand high temperatures and stresses, which are experienced, for example, in automotive and gas turbine components.
An ultra-fine beta silicon carbide powder is desirable in order to minimize shrinkage and optimize sinterability.
High purity silicon carbide in this specification including the claims refers to a product containing in excess of 99.5% by weight of silicon carbide.
Although a number of processes for the production of silicon carbide have been disclosed in prior art, none appear to have the potential of producing large scale quantities of low cost, high quality powders. Existing processes for making silicon carbide start with a mixture of either elemental silicon or silicon dioxide (silica) and carbon. It is preferable to manufacture silicon carbide using silica as a raw material because it is readily available in reasonably pure form at significantly less cost than elemental silicon.
Much of the silicon carbide produced today is manufactured by the Acheson process. This process involves mixing silica sand and carbon from various sources and reacting them in an electrical furnace at an extremely high temperature of over 2400.degree. C., to produce coarse, highly crystallized, mostly alpha-phase silicon carbide. The manufacture of a fine, sinterable powder requires a series of crushing and milling operations, each followed by acid cleaning to remove metal contamination. The average particle size of silicon carbide obtained by the milling may be as low as ten microns, but for engine and other advanced technology applications, still finer powders are required. The process of milling to reach micron and sub-micron size powders greatly increases the production costs and makes the Acheson process economically unacceptable.
U.S. Pat. No. 4,276,275 describes a method for making fine powder by reacting a carbon electrode with expensive elemental silicon. The equipment is a vacuum chamber pressured with an inert gas and the product is collected by scraping the walls of the vessel.
Another method of producing beta silicon carbide is described in U.S. Pat. No. 4,368,181. In this method, since the starting mix of raw materials does not have excess carbon to react with gaseous silicon monoxide (SiO) and thus to prevent excessive silica losses, a two-temperature zone furnace had to be designed. Highly volatile silicon monoxide formed in the high temperature zone is condensed in the low temperature zone and is then converted into beta-phase silicon carbide in the next firing cycle. To produce a powder of (relatively) high silicon carbide content, repeated re-firing operations are necessary. The analysis of the product listed in the examples in this patent show the presence of unreacted silica. This suggests that this process is not suitable for the production of a fully reacted powder.
In many of the earlier processes, the removal of carbon monoxide gas is achieved by non-reactive gas flushing, venting to atmosphere or applying a vacuum. None of the earlier processes teach the use of extremely low absolute pressures of less than 500 millitorr, as is taught by the present invention. On the contrary, an early proposal in Canadian Patent No. 782,522 (Dietz) teaches that too great a vacuum should not be applied to the system as it will result in the decomposition of the silicon carbide product according to the reaction: EQU 2SiO.sub.2 +SiC.fwdarw.3SiO+CO
It further specifies that the partial pressure of carbon monoxide should be kept within limits determined by temperature. Process conditions employed by Dietz did not result in complete conversion to silicon carbide as shown by the need to remove silica by hydrofluoric acid leaching.
German Published Patent Application No. 2,848,377 discloses a process for making beta-phase silicon carbide of surface area greater than 5m.sup.2 /g from silicic acid and acetylene black. The disclosure is very specific as to the type of raw materials utilized, both the silicic acid powder and carbon powder are required to have a surface area of at least 100m.sup.2 /g and the silicic acid has to be of a highly dispersed type. The patent describes a specific way of mixing and kneading the raw materials with the presence of a large amount of water, which must be removed by drying prior to processing. The patent also stresses the need to mold the mixed wet raw materials in order to obtain the required surface area of the powder. The dried, extruded shapes are heated to 1300.degree.-1700.degree. C. in either a flowing inert gas or a vacuum of less than 75.times.10.sup.3 millitorr (100 millibars).
Silicic acid powder and carbon powder having surface areas in the range of 100m.sup.2 /g would correspond to powders having particle sizes in the range of 10.sup.-2 microns or less. These are extremely fine powders and would be quite costly as raw materials. It has been found that a vacuum of 60-100mbar is not sufficient to promote the complete conversion of silica or silicic acid to silicon carbide, where carbon having a particle size of 0.3 microns is used either in combination with silicic acid having a surface area of 100m.sup.2 /g or coarser silica having a particle size of 45 microns. A review of the examples in this German Patent No. 2,848,377 indicates that even using the extremely fine powders suggested, the product contains unreacted feedstock and it contains less than 99.5% silicon carbide. Furthermore, it has been found that the application of a vacuum of 100mbar during the heating up period causes significant material losses as silicon monoxide forms a vapour at a temperature below that of silicon carbide formation and this vapour is drawn off by the vacuum source.